1. Field of the Invention
The present invention relates to a receiving apparatus that can improve the S/N ratio of a receiving signal and a receiving antenna for an ultra long wavelength electromagnetic wave used for transmitting downhole information onto the ground in the case of drilling oil wells or gas wells.
2. Description of the Prior Art
When drilling an oil well or a gas well, it is essential to obtain downhole information on geological characteristics such as downhole temperature, downhole pressure and so on. There are several conventional methods to measure such information. One method is to measure by placing a well logging instrument into the ground from the top of the drilled well after having lifted a drill pipe on the ground. Another method is to measure components of drilling fluid called "mud" which is circulated via a drill pipe between the downhole and the surface during the drilling, which method is called mud logging method.
However, it takes a long time for measuring downhole information by these methods, and hence, real time measurement of the downhole information is impossible. For this reason, measuring technique aiming at the real time measurement, which is called MWD (Measurement While Drilling), has been studied recently, and various methods are proposed. Above all, a technique employing an electromagnetic wave has been attracting attention.
For example, FIG. 1 shows an arrangement of a conventional receiving apparatus disclosed by a U.S. reference "Status report: MWD Technology" in PETROLEUM ENGINEER International, OCTOBER, 1988. In this figure, a drilling rig 2 is built on the ground 1. Immediately under the drilling rig 2, an oil well 3 is formed by drilling. Near the top of the oil well 3, there is provided a casing pipe 4 made of steel so as to prevent the wall of the oil well from collapsing. In addition, a drill pipe 5 extends in and projects out of the casing pipe 4. To the tip of the drill pipe 5, a drill collar 7 is attached via an insulating collar 6. At the top of the drill pipe 5, there is provide a blow out preventer 5a on the ground. At the tip of the drill collar 7, a bit 8 for drilling is joined. Thus, by rotating the drill pipe 5 with a motor 14 via chains or gears, the ground 1 is drilled. In the drill collar 7, a transmitting apparatus 9 is incorporated to transmit downhole information onto the ground. The transmitting apparatus detects temperature, pressure, or the like of the downhole, converts them into electric signals, and sends the information in the form of modulated signals. Transmitting output terminals of the transmitting apparatus 9 are connected to the drill pipe 5 and the drill collar 7, respectively, which are interconnected via the insulating collar 6. Thus, the drill pipe 5 and the drill collar 7 serve as a transmitting dipole antenna for sending a modulated ultra long wavelength electromagnetic wave onto the ground.
On the other hand, at the proximal portion of the drilling rig 2, there is provided a dipole type receiving antenna 11 for detecting the ultra long wavelength electromagnetic wave transmitted from the downhole. One terminal of the receiving antenna 11 is led out of the casing pipe 4, and the other terminal thereof is composed of an electrode 10 buried in the ground. Thus, the ultra long wavelength electromagnetic wave is received by the antenna. A signal received by the receiving antenna 11 is inputted to an amplifier 12, and the amplified signal is inputted to a signal processor 13 in order to obtain downhole information by decoding the signal. As an ultra long wavelength electromagnetic wave, a wave whose frequency is on the order of tens of hertz is used depending on stratum, geology and depth.
A current i.sub.s flows through the drill pipe 5 and the ground when an electromagnetic wave is sent into the ground by the transmitter 9, as shown by solid lines in this figure. Here, E represents equipotential lines formed by the current i.sub.s. The receiving antenna 11 detects the difference of the potential. The detected signal is amplified by the amplifier 12, and is recognized by the signal processor 13 as downhole information.
The drill pipe 5 is directly rotated by the motor 14 so that the drilling rig 2 rotates the bit 8 to drill. When the motor 14 rotates, ground currents in.sub.1, in.sub.2 and in.sub.3 flow through stray capacitances C.sub.1, C.sub.2 and C.sub.3 between lead lines and the ground as shown in FIG. 2, or between windings of the motor 14 and the drilling rig 2. The ground currents flow into the ground 1 through the drill pipe 5, muddy water 15 indicated by oblique lines in FIG. 6, and the casing pipe 4, as shown by broken lines i.sub.n in FIG. 1. Thus, the currents interfere with the transmission signal i.sub.s, and decrease the S/N ratio of the transmission signal i.sub.s. The current i.sub.n is called a noise current.
Since the conventional receiving apparatus is arranged as described above, ground currents flow out when the electric machines (especially the motor) installed on the floor of the drilling rig are in operation. The currents interfere with a signal received by the antenna via the ground so that the S/N of the received signal from the ground decreases. This presents problems that information transmission from deep locations is hindered, and that the reliability of the information reduces.
On the other hand, as a noise canceller for canceling noises mixed with an input signal to such a receiving apparatus, a noise canceller using an adaptive filter is known.
FIG. 3 is a block diagram showing a conventional noise canceller using a parallel adaptive filter disclosed by B. Widrow et al., in "adaptive filters and neural networks for adaptive pattern recognition", in Nikkei Electronics, 1988, 9, 5 (No. 455), pp. 201-218. In this figure, reference numeral 31 denotes an adaptive filter, and reference numeral 32 designates a subtracter. The adaptive filter 31 receives, as a reference signal, a noise source signal n'(n) that has large correlation with a noise n(n) entering a main input signal S(n), adjusts the filtering characteristic of itself on the basis of an output signal .epsilon.(n) of the subtracter 32 so that the noise n(n) mixed with the main input signal S(n) is canceled, and supplies the subtracter 32 with a signal n(n). The subtracter 32 subtracts the output signal n(n) of the adaptive filter 31 from the main input signal S(n)+n(n) which includes the noise n(n).
Next, the operation is explained.
As a main input to the noise canceller, a signal S(n) with which a noise n(n) is mixed is inputted to the subtracter 32. In addition, a noise source signal n'(n) having strong correlation with the noise n(n) is inputted to the adaptive filter 31 as a reference signal. The adaptive filter 31, adjusting its filtering characteristics as explained later, converts the noise source signal n'(n) into n(n), which is subtracted by the subtracter 32 from the main input signal S(n)+n(n) which includes the noise n(n). The adaptive filter 31 adjusts its filtering frequency characteristics so that a mean square &lt;.epsilon..sup.2 (n)&gt; becomes minimum on the basis of a subtraction result .epsilon.(n) of the subtracter 32. As a result, the input-output relation of the adaptive filter 31 is continually adjusted in such a manner that the filter outputs an optimum estimate waveform n(n) equal to the noise n(n) mixed with the main input signal S(n) when the noise source signal n'(n) is inputted as the reference signal. The filtering characteristics are adjusted in accordance with the following algorithm.
The result .epsilon.(n) of the subtraction can be expressed by the following equation: EQU .epsilon.(n)=S(n)+n(n)-n(n)
In this case, the mean square of the subtraction becomes, ##EQU1## Generally, a signal and a noise have no correlation, and hence, the following approximation holds: EQU &lt;S(n){n(n)-n(n)}&gt;=0
Thus, equation (1) can be rewritten as EQU &lt;.epsilon..sup.2 (n)&gt;=&lt;S.sup.2 (n)&gt;+&lt;{n(n)-n(n) }.sup.2&gt; ( 2)
Equation (2) shows that varying n'(n) so that the square means &lt;.epsilon.(n)&gt; takes a minimum value does no effect on the square mean of the signal. In addition, when &lt;.epsilon..sup.2 (n)&gt; is minimum, &lt;{n(n)-n(n)}.sup.2 &gt; is minimum, and n(n) becomes an optimum approximation of n(n). As a result, .epsilon.(n) becomes an approximation of S(n).
A conventional noise canceller using an adaptive filter is arranged as described above, and cancels noises with reference to a single noise source signal. As a result, when noises generated by a plurality of noise sources enter a signal, sufficient noise cancelling cannot be achieved, resulting in increase in residual noises.